Process for producing nucleation and for controlling grain size in ingots and castings



y 30, 1967 c. JOHNSTON ETAL 3,322,183

PROCESS FOR PRODUCING NUCLEATION AND FOR CONTROLLING GRAIN SIZE IN INGOTS AND CASTINGS Filed June 9, 1964 United States Patent Ofifice PROCESS FOR PRODUCING NUCLEATION AND FOR CONTROLLING GRAIN SIZE IN INGOTS AND CASTINGS William 'C. Johnston and Gerald R. Kotler, Pittsburgh, and William A. Tiller, Export, Pa., assignors, by mesne assignments, to the United States of America as represented by the Secretary of the Air Force Filed June 9, 1964, Ser. No. 373,890 4 Claims. (Cl. 164-49) This invention concerns a process for casting molten metals with controlled nucleation, grain size, and distribution in the production of castings, ingots, and the like.

In the production of metal shapes by pouring molten metal into molds Where the metal solidifies, continuous effort is being made to improve the characteristics of the solidified metal. Columnar grains with long and continuous grain boundaries between adjacent grains are objectionable since in metal rolling processes weaknesses may occur along the grain boundaries.

Centrifugal casting improves the resulting macrostructure of the metal grains in structure, in density and in physical characteristics.

The grain structure of a steel ingot teemed into a metal mold and refined in avoiding the formation of large columnar grains by the electromagnetic stirring of the molten steel just prior to its solidification, i described in Transactions of the Metallurgical Society of the American Institute of Metallurgical Engineering, volume 221, October 1961, pages 993-1001. The stirring device actuated by the circuitry shown in block diagram on page 994 produces comparative metal macrostructures illustrated on pages 995-1000 inclusive, of the article. Grain refined ingots free of surface cracks from upset forging, are illustrated on page 999 and are described on pages 1000-4001.

The direction of the stirring of the molten metal may be reversed periodically for more uniform structure if desired, as is disclosed in the Patent No. 3,079,246, issued Feb. 26, 1963, to Elbert C. Smith. Metal grain structure also has been improved by mechanical vibration at ultrasonic frequencies by the introduction into the melt of nucleating agents and the like. The Iron Age, for Sept. 8, 1960, contains an article, at pages 102-104, that pertains to advantages that are derived from magnetic stirring procedures.

The object of the present invention is to provide a new and improved process for imparting rotary motion to molten metal during its transistion from the liquid to the solid physical state, with an o timum number of nucleation centers imparting a shower effect that is apparent in the magnification of polished .and etched surfaces of metal castings so produced.

The present invention developed from work of which a part was published in the Transactions of the Metallurgical Society of the American Institute of Metallurgical Engineering, vol. 227, August 1963, pages 890-896 inclusive. The apparatus used is a part of the present invention and appears on page 892 of the publication and is represented in the accompanying drawings.

In the drawings:

'FIG. 1 is a perspective View partly cut away and in section of the apparatus that is employed in performing the process of the present invention;

FIG. 2 is -a perspective view of the melt container part ofthe apparatus in FIG. 1;

FIG. 3 is a perspective view of the stirring coils removed from the assembly in FIG. 1; and

FIG. 4 is an exploded fragmentary view the bolt and insulation part of the coils in FIG. 3.

The apparatus in the accompanying drawings comprises a container 1 for a specimen 2 supported through 3,322,183 Patented May 30, 1967 an evacuation tube 3 by a cable 4, for positioning the melt centrally of a set of heating and stirring coils 5, or for quenching the melt in a tank 6 that contains a desired coolant, such as ice water.

The tubular sample container 11 is shown as being provided with an axially extending thermocouple hous ing shield 10 that is sealed at its upper end through the container wall to receive a thermocouple 11 therein. The hot junction of the thermocouple is positioned about centrally of the molten specimen 2.

The upper end of the sample container 1 is open to receive powdered metal or the like, and makes a ground glass joint with the lower end of the evacuation tube 3. Under vacuum, the joint has ample adhesion for lifting and lowering the container 1 and its contents by the operation of a cable 4. The evacuation tube 3 opens into a vacuum tube connection arm 12 that connects with a vacuum pump.

The sample container 1 is adapted for travel axially and centrally of the heating and stirring assembly. The assembly illustratively comprises an upper ambient heater 15, a lower ambient heater 16 housing a coil 17, and a desired plurality of stirring coils therebetween.

The stirring coils illustratively comprise the six copper split rings 20 through 25, that are plated with nickel and then with gold and .that are insulated from each other 'axially by mica-spacers, of which the spacer 30 shown in the drawing is illustrative. The rings are secured together by suitable means such as by bolts of which the bolt 35 shown in FIG. 4 is illustrative, with each bolt shaft in an alumina sleeve 36. Each. alumina sleeve 36 is mounted within a mica sleeve 37. One electrically conducting copper bar 38 connects the coils 22 and 25. Another electrically conducting copper bar 40 connects the coils 2'1 and 24. In a similar way, another copper bar, not shown, connects the coils 20 and 23.

The rings 20 through 25 are energized with direct current energy or with alternating current energy, as preferred, by high current carrying cables 42, 42, 42", 42, 42"" and 42, that are attached to the rings in a suitable Way such as by stainless steel bolt-s or the like. The rings 20 through 25 preferably are plated with nickel to reduce the diffusion of copper from the rings to form copper oxide, and then are plated with gold to provide low resistance contacts. A transite base 45 supports the group of stirring coils and is interposed between the stirring coils and the lower ambient heater 16. The upper and the lower ambient heaters I15 and 16 are energized by current supplying leads, not shown.

The apparatus is used by first opening the ground glass seal between the container 1 and the evacuation tube 3.

The desired gram sample of charge specimen 2 is in serted into the container 1 and the seal is closed. The atmosphere above the charge 2 is removed through the evacuation tube 3 and the cable 4 is adjusted to position the charge 2 about centrally of the coil assembly. The heaters 15 and 16 are maintained energized with the thermocouple L1 indicating the molten state of the specimen 2 for a desired period of time.

High current of direct current or alternating current energy as preferred, is then impressed on the stirring coils 20 through 25 and the nuclei dispersed phase causes a fine particle shower throughout the liquid dispersing medium. Each coil if preferred, may consist of about turns of wire.

The de-energization of the heating coils 15 and 16 causes the lowering of the temperature of the liquid phase as the liquid solidifies, thereby fixing the fine particle shower distribution uniformly throughout the matrix.

The abrupt chilling of the melt 2 is provided for by operation of the cable 4 dropping the container 1 into a desired coolant, of which ice water is illustrative, within the quenching tank 6.

The electromagnetic stirring device shown in the accompanying drawings may be used for continuous castings, chill castings or for other types of castings.

In the continuous castings and the chill castings, an interface is advancing into the melt and to induce a fine grain structure, it is necessary that the melt be stirred throughout the freezing process. If the stirring is interrupted, the interface will become columnar and the grains will enlarge.

The desirable fine grain structure, forming during casting, is characterized by the shower effect. The stirring coils placed around the sample may be either embedded in the mold or may be arranged as a separate piece of apparatus. When the mold with its molten sample is cooled below the melting temperature of the melt, super cooling is produced until the temperature of recalescence below the melting point is arrived at with the passage of time.

The stirring preferably is star-ted at the recalescence temperature and continues for the short period of time during which the temperature rises to the melting point of the melt. For an alloy, the stirring time in a similar manner follows the eutectic cooling curve back up to the melting point of the alloy.

In general, the larger the under cooling or the farther below the cooling curve the recalescence point occurs, and the stirring operation is started, the greater number of grains per unit volume in the shower effect will be produced. The number of grains produced in the shower effect depends both on the under cooling of the melt at which the stirring operation is started, and on the field strength the coils apply to the melt.

F or melts of materials which do not under cool to temperatures of recalescence and at which stirring is started, as much as other materials increasing the magnitude of the melt stirring magnetic flux, compensates for the lack of under cooling in producing comparable numbers of grains in the shower effect.

Stirring of the melt only needs to be applied during a short part of the cooling process in some cases, to produce adequate fine grain results. Stirring initiated shortly after the beginning of recalescence and continued to the arrival of the melting point temperature results in several orders of magnitude increase in the number of fine grains in the shower effect that results. Stirring preferably begins at the recalescence temperature below the cooling curve and is stopped before the melt arrives at the melting point temperature, where solidification occurs and the cooling curve is renewed.

An oscillating magnetic field is imposed on the melt when the melt is concentrically surrounded by a single coil about 2 inches wide and composed of 150 turns of wire, through which is passed a single phase 400 cycle per second applied power. This field produces small r-adio oscillations in the liquid melt volume. Cooling the alloy melt specimen under these conditions results in an increase of several orders of magnitude of fine grains in the shower effect accomplished as compared with a zero magnetic field performance.

It is to be understood that the steps of the process and the apparatus that are described herein, are submitted as being illustrative of a successfully operative embodiment of the present invention and that limited modifications may be made therein within the scope of the present invention.

We claim:

1. The process for producing nucleation and for controlling grain size in ingots and in castings by positioning a container of powdered solid metal substantially centrally of a plurality of split ring stirring coils between upper and lower ambient heaters, liquefying the metal in the container by the energization of the ambient heaters, electromagnetically stirring the liquid metal in the container by the energization of the stirring coils to produce small radio oscillations in the liquid melt volume during the formation throughout the liquefied metal of nucleating centers, and rigidizing the distribution of the nucleating centers throughout the metal by abruptly reducing the temperature of the metal in the container.

2. The process described in claim 1, wherein the nucleating centers produce a shower effect by their profusion in substantially maximum numbers at the temperature of solidification.

3. The process described in claim '1, wherein the melt has a reoalescence temperature below its melting point about at which recalescence temperature the electromagnetic stirring of the liquid metal is caused to start and is continued to a temperature slightly below the melting point of the melt.

4. The process described in claim 1, wherein the electromagnetic stirring of the liquid metal is accomplished within a temperature range below the melting point of the liquid metal and during the metal recalescence.

References Cited UNITED STATES PATENTS 2,013,653 9/1935 Hoke 22-200 X 2,083,022 6/1937 Hoke 22-57 2,848,775 7/1958 Ettenreich 14812.9 2,963,758 12/1960 Pestel et al. 22-2001 X 3,153,820 10/1964 Criner 22--2l 6 X 3,246,373 4/1966 Lyman 22-200 X FOREIGN PATENTS 699,156 10/1953 Great Britain.

OTHER REFERENCES Magnetic Stirring: A New Way To Refine Metal Structure, Crossley, The Iron Age, Sept. 8, 1960, pp. 102-104, TS 200.18.

WILLIAM J. STEPHENSON, Primary Examiner. R. S. ANNEAR, Assistant Examiner. 

1. THE PROCESS FOR PRODUCING NUCLEATION AND FOR CONTROLLING GRAIN SIZE IN INGOTS AND IN CASTINGS BY POSITIONING A CONTAINER OF POWDERED SOLID METAL SUBSTANTILLY CENTRALLY OF A PLURALITY OF SPLIT RING STIRRING COILS BETWEEN UPPER AND LOWER AMBIENT HEATERS, LIQUEFYING THE METAL IN THE CONTAINER BY THE ENERGIZATION OF THE AMBIENT HEATERS, ELECTROMAGNETICALLY STIRRING THE LIQUID METAL IN THE CONTAINER BY THE ENERGIZATION OF THE STIRRING COILS TO PRODUCE SMALL RADIO OSCILLATIONS IN THE LIQUID MELT VOLUME DURING THE FORMATION THROUGHOUT THE LIQUEFIED METAL OF 